An optical fiber which is used for optical communication shows a characteristics of a lower transmission loss in addition to its larger bandwidth compared with other transmission lines such as a copper wire, a coaxial cable, etc. Nevertheless, the transmission loss of the optical fiber can not be completely disregarded, and thus an optical signal which is transmitted should be periodically amplified in order to compensate for the attenuation of the signal. Such an amplification of the optical signal is performed by use of repeaters inserted between the fibers.
In most of optical communication systems currently being used, the repeater is constituted by a detector, an electrical amplifier and a semiconductor laser. In such a repeater, the detector transforms an attenuated optical signal into an electrical signal, the amplifier amplifies the transformed electrical signal, and the semiconductor laser is driven by the amplified signal to transmit a new optical signal to the next stage. However, the repeater has disadvantages in that it increases noise in the signal and the speed of transformations between the optical signal and the electrical signal are restricted by the bandwidth of components such as the detector and the amplifier.
Thus, a pure optical amplifier for amplifying an optical signal as itself has been developed and is being used. Furthermore, such an optical amplifier is used not only for optical communications but also for power amplification for a low-power optical source, signal splitting compensation in a cable TV network, or preamplification with respect to an optical detector.
The most dominating optical amplifier is an Erbium-doped fiber amplifier (hereinafter referred to as "EDFA"), which shows a high gain of 40 dB or above, a high output power, and a low noise figure in a band near 1.55 .mu.m wavelength.
FIG. 1 is a block diagram of a typical EDFA, wherein FIG. 1a shows a forward amplifier and FIG. 1b shows a reverse amplifier.
The forward amplifier of FIG. 1a includes a first lens 10 for focusing an input light emitted from a first optical fiber (not shown), an optical detector 11 for detecting the intensity of the input light, an optical splitter 12 for coupling the optical detector 11 on a transmission path, a first isolator 14 for enabling an optical signal to flow in only forward direction, a laser diode 16 for generating an optical signal for pumping, a coupler 18 for coupling the laser diode 16 on the transmission path, an Erbium-doped fiber (hereinafter referred to as "EDF") for amplifying an input optical signal through a stimulated emission by use of photons generated by the pumping operation of the laser diode 16, a second isolator 22 for enabling the optical signal to flow only in the forward direction, an optical detector 24 for detecting the intensity of an output light, an optical splitter 26 for coupling the optical detector 24 on the transmission path, and a second lens 28 for focusing the output light to output the focused light to a second optical fiber (not shown).
In the forward amplifier having such a configuration, the EDF 20 is formed by doping the core of an optical fiber with Erbium through a modified chemical vapor deposition (CVD) method using an source gas such as Erbium trichloride (ErCl.sub.3), and has an emission wavelength of 1.536 .mu.m.
Meanwhile, the laser diode 16 generates a laser light having a wavelength of 1.48 .mu.m or 980 nm and provides the laser light to the EDF 18. The laser light pumps electrons of Erbium to cause a distribution inversion, so that the EDF 18 outputs a laser light having a wavelength of 1.536 .mu.m.
Of two isolators 14 and 22, the first isolator 14 prevents a degradation amplification efficiency which may results from the propagating of the light amplified in the EDF 20 or a spontaneously emitted light in the reverse direction. The second isolator 22 prevents the optical signal from being reflected by a connector (not shown) at an output port and so on and entering into the EDF 20.
The reverse amplifier of FIG. 1b has the same configuration as that of the forward amplifier of FIG. 1a except that the pumping laser diode 17 is coupled to the rear side of the EDF 21 by the coupler 19.
Meanwhile, U.S. Pat. No. 4,548,478 issued Oct. 22 1985 to Masakata Shirasaki and entitled "OPTICAL DEVICE" describes an optical isolator.
FIG. 2 illustrates the optical isolator disclosed by Shirasaki, which is employed in an optical amplifier. The optical amplifier, which is similar to that shown in FIG. 1, includes a first lens 31 for focusing an input light emitted from a first optical fiber (not shown), an optical detector 32 for detecting the intensity of the input light, an optical splitter 34 for coupling the optical detector 32 on a transmission path, an isolator 36 for enabling an optical signal to propagate only in one direction.
The optical splitter 34, which is implemented using a prism or an optical coating, separates the optical signal received therein to output some of the optical signal to the optical detector 32 and the remaining signal to the isolator 36.
The isolator 36, which was disclosed by Shirasaki, consists of two tapered plates 37 and 39 which are made of birefringent materials such as rectile and calcite and a 45.degree. Faraday Rotator 38 interposed between the tapered plates 37 and 39.
However, the isolator 36 brings about polarization mode dispersion arising from the difference in refractive index or propagation velocity of lights. Therefore, a compensator 40 shown in FIG. 2 is additionally included to compensate for the polarization mode dispersion, which is described in European patent application published with number of 533,398 A1 and assigned to AT&T Bell Laboratories.
Further, the conventional optical amplifier has so many components that the structure is complicated and insertion loss is large. Also, as shown in FIG. 2, optical fibers should be spliced in many places such as between the optical splitter 34 and the optical detector 32, between the optical splitter 34 and the isolator 36, between the isolator 36 and the compensator 40, etc. Consequently, the manufacturing process is complicated whereby the unit cost of a product increases Meanwhile, since the light is incident on the optical splitter at an incident angle of 45.degree., a large polarization dependent loss is resulted in.